The following relates to combustion of waste products, biomass materials, and other combustible materials, and to a fuel feeder for feeding such products or materials into a fluidized bed boiler, such as a bubbling fluidized bed (BFB) or circulating fluidized bed (CFB) boiler, or for a stoker-fired unit, and to fluidized bed boilers for combusting such products and materials which employ such a feeder, and to related arts.
During combustion, the chemical energy in a fuel is converted to thermal heat inside the furnace of a boiler. The thermal heat is captured through heat-absorbing surfaces in the boiler to produce steam. Fuels used in the furnace include a wide range of solid, liquid, and gaseous substances. Combustion transforms the fuel into a large number of chemical compounds. In some applications, solid biomass waste byproducts are used as fuel for the fluidized bed boiler.
Fluidized bed boilers are one way to burn solid fuels. Generally speaking, a fluidized bed boiler includes a bed formed from a stacked height of solid particles. A fluidization gas distribution grid, such as an open bottom system or a flat floor system, is located beneath the bed. An open bottom system is characterized by widely spaced distribution ducts on which are mounted air bubble caps for distributing fluidizing gas (typically air) under pressure to fluidize the bed. In a flat floor system, the distribution ducts form the floor of the boiler. At sufficient gas velocities, the solid particles exhibit liquid-like properties.
With reference to FIG. 1, an illustrative bubbling fluidized-bed (BFB) boiler 8 of a known design (available from Babcock & Wilcox Power Generation Group, Barberton, Ohio, USA) includes a bubbling bed 10 onto which fuel 12 is delivered via a feeder 14 comprising an air-swept spout. The fluidized bed 10 suitably comprises solid particles such as, for example, sand. A gas-tight furnace flue (only the lower portion of which is shown in FIG. 1) includes gas-tight water cooled walls 16, 17. Air is introduced into the bubbling bed 10 through air ducts 18, and spaced-apart bubble caps 20 facilitate removal of large tramp material. In an underbed ash removal system 22, tramp material moves downward and cools before being removed through bottom hoppers 24 onto a suitable conveyor system or the like (not shown). Heat from combustion on the fluidized bed 10 heats water in the gas flue wall pipes 16, 17 which may drive a steam generator or other useful work. In some embodiments water in the gas flue pipes 16, 17 flows in a closed-loop recirculation path (usually including a make-up water line). The illustrative BFB boiler 8 of FIG. 1 is merely an example, and the disclosed feeder systems and other disclosed aspects are readily incorporated into fluidized bed boilers of various designs, such as BFB and CFB designs, stoker-fired units, or so forth. In some embodiments, the feeder 14 may pass through a non-water cooled refractory furnace wall (e.g., a brick furnace wall) rather than through tube wall 16 as in the illustrative embodiment of FIG. 1, or through any other type of boiler wall. It is contemplated for the furnace wall through which the feeder 14 passes to include additional features such as thermal insulation material, an outer casing, or so forth.
Such boilers can process a wide range of waste materials, including wood waste, bark, paper mill sludge, recycled paper, sewage sludge, and various biomass materials such as stillage left over from corn (or other biomass) ethanol production. For BFB boiler operation, the feeder 14 should deliver the fuel 12 to the bed 10 without being burned in suspension. This is relatively straightforward to achieve for solid fuel, but is more difficult to achieve with liquid fuel, especially if it is atomized to increase its surface area. An existing approach for processing liquid waste is to incorporate it into solid waste prior to delivery of the liquid/solid mixture to the BFB boiler. However, this approach complicates material handling as the wet mixture can be difficult to move and handle. Another existing approach is to inject the liquid directly into the bubbling bed, typically from the floor. This approach can generate non-uniform concentrations and chemistry over the bed, which can lead to agglomeration of the bed material.
It would be desirable to provide the ability to use multiple fuel sources in a boiler, particularly a fluidized bed boiler. This would expand the range of fuels that could be used.